A monitoring system in which a camera to photograph inside of a store is installed and a situation inside the store is monitored using images of the camera has been widely distributed in an eating and drinking place such as a restaurant and a coffee shop. It is possible to grasp a situation of customers who visited the store using the images of the camera, particularly, a group configuration of the customers, that is, whether the customers visited the store in a single person, whether the customers visited the store in a group, or how many customers visited the store in a case where the customers visited the store in a group. A store operator can consider, for example, improvements for efficiently operating the store, such as change of a layout of tables or seats based on the pieces of information, and it is useful for improving sales and profits of the store.
As matters related to a technique for grasping such a group configuration of persons, conventionally, a technique in which a distance between persons is obtained from an image in which a plurality of persons are photographed and a group configuration of persons is determined based on the distance is known (see PTL 1 and PTL 2).